The demand curve facing the monopolist is given below:

We first compute the total revenue and marginal revenue at different level of production:

The monopolist will produce until marginal revenue is not lower than the marginal cost. At Q = 4, the marginal revenue is $18, which is higher than the marginal cost of $15. At Q = 5, the marginal revenue is $1, which is lower than the marginal cost. Thus the profit-maximizing level of production is Q = 4.